CaTiO3 Interfacial template structure on semiconductor-based material and the growth of electroceramic thin-films in the perovskite class

ABSTRACT

A structure including a film of a desired perovskite oxide which overlies and is fully commensurate with the material surface of a semiconductor-based substrate and an associated process for constructing the structure involves the build up of an interfacial template film of perovskite between the material surface and the desired perovskite film. The lattice parameters of the material surface and the perovskite of the template film are taken into account so that during the growth of the perovskite template film upon the material surface, the orientation of the perovskite of the template is rotated 45° with respect to the orientation of the underlying material surface and thereby effects a transition in the lattice structure from fcc (of the semiconductor-based material) to the simple cubic lattice structure of perovskite while the fully commensurate periodicity between the perovskite template film and the underlying material surface is maintained. The film-growth techniques of the invention can be used to fabricate solid state electrical components wherein a perovskite film is built up upon a semiconductor-based material and the perovskite film is adapted to exhibit ferroelectric, piezoelectric, pyroelectric, electro-optic or large dielectric properties during use of the component.

BACKGROUND OF THE INVENTION

This invention relates generally to structures and the preparation ofsuch structures for use in semiconductor and related applications andrelates, more particularly, to the growth of epitaxial thin films uponsemiconductor-based materials in the Group III-V, IV and II-VI classessuch as, by way of example and not limitation, silicon orsilicon-germanium alloys.

Electroceramic thin-films and, in particular, ferroelectric oxides areknown to support the phenomenon of ferroelectricity and are believed tobe useful in a wide range of applications such as nonvolatile memories,optical waveguides, and as a capacitor material in random accessmemories (RAM), dynamic random access memories (DRAM), electricallyprogrammable read only memories (EPROM) and the like. For example, inepitaxially grown ferroelectric oxide layers wherein thecrystallographic orientation of the layers is ordered, the orientationof the ferroelectric dipole moment is the basis for logic-stateretention in nonvolatile memories. Thus, it would be desirable tointegrate a ferroelectric oxide with a semiconductor-based substratecomprised, for example, of silicon or silicon-germanium to render amonolithic structure which possesses both semiconductor andferroelectric properties.

In solid state electrical devices of the prior art, such asferroelectric field effect transistors (FFETs) and capacitors orinactive gate transistors which incorporate a semiconductor material anda ferroelectric material, such as a perovskite having the generalformula ABO₃, the devices are incapable of taking appreciable advantageof the ferroelectric and/or dielectric properties of the ferroelectricmaterial. For example, the FFETs constructed to date have beenunsatisfactory in performance, and the capacitors and inactive gatetransistors constructed to date have been too leaky and thus incapableof holding a charge for a lengthy period of time. It would therefore bedesirable to provide a solid state electrical device of this class whichtakes appreciable advantage of the ferroelectric and/or dielectricproperties of the ferroelectric material incorporated therein.

Accordingly, it is an object of the present invention to provide a newand improved structure comprised of a crystalline electroceramicthin-film and a semiconductor-based substrate and a process for growingthe thin-film upon the substrate.

Another object of the present invention is to provide such a structurewhich includes an ABO₃ material such as, by way of example and notlimitation, a perovskite, and in particular a perovskite in the BaTiO₃class, grown upon materials selected from the Group III-V, IV or II-VIclasses of materials including, by way of example and not limitation, asilicon or silicon-germanium substrate wherein the grown perovskite isepitaxial and fully commensurate with the underlying material upon whichit is grown.

Still another object of the present invention is to provide such astructure which utilizes a template structure interposed between thematerial surface of the Group III-V, IV or II-VI material forming thesubstrate and the desired ABO₃ material such as a perovskite forfacilitating the fully commensurate growth of the desired ABO₃ materialupon the substrate.

A further object of the present invention is to provide a new andimproved solid state electrical component including a material adaptedto exhibit ferroelectric, piezoelectric, pyroelectric, electro-optic orlarge dielectric properties during use of the component.

SUMMARY OF THE INVENTION

This invention resides in a monolithic crystalline structure and aprocess for growing an ABO₃ material, such as a perovskite, film ontothe surface of a Group III, IV or II-VI semiconductor-based materialwherein the material surface provided is, by way of example and notlimitation, a face-centered-cubic (fcc) lattice structure like that ofsilicon or silicon-germanium.

The ABO₃ material has a lattice parameter which matches thesemiconductor surface cube on cube or which closely approximates thequotient of the lattice parameter of the semiconductor surface dividedby the square root of 2.0 and further has a crystalline form comprisedof two constituent metal oxide planes comprised of AO and BO₂,respectively. When the metal elements A and B of crystalline form of theABO₃ material are compared to one another, the element A provides alarge cation in the crystalline structure of the ABO₃ material, and theelement B provides a small cation in the crystalline structure of theABO₃ material.

For example, in an ABO₃ material wherein the element B is the metalTitanium (Ti) (so that the BO₂ constituent plane is TiO₂), the Ti metalof the TiO₂ plane provides a small cation in the crystalline structureof the ABO₃ material, and the metal oxide of the constituent metal oxideplane AO includes the metal element A which provides the large cation inthe crystalline structure of the ABO₃ material. In order to ensurecommensurate periodicity during the buildup of the ABO₃ material, theformation of a single plane layer consisting of a metal oxide (such as,for example, AO) provided with a large cation is immediately followed bythe deposition of a single plane layer consisting of a constituent metaloxide plane BO₂, rather than the constituent metal oxide plane AO.

In addition, the ABO₃ material of the epitaxial film is arranged uponthe semiconductor surface so that a first single plane consisting of theoxide constituent AO is fully epitaxial and fully commensurate with thesurface of the substrate, and a second single plane consisting of theother of the two constituent metal oxide planes (i.e. the oxide plane ofBO₂) of the crystalline structure of the ABO₃ material is fullycommensurate with the first single plane of AO and wherein theorientation of the ABO₃ material of the film is matched either cube oncube with the lattice structure of the substrate or is rotated 45° withrespect to the orientation of the material surface of the substrate.

A process of the invention includes the steps of providing a substrateof semiconductor-based material having a surface which is provided by anfcc lattice structure like that of silicon or silicon-germanium, andpositioning the substrate within an oxygen-free environment in anultra-high vacuum facility. Then, an alkaline earth oxide is selectedwhich has a lattice parameter which closely approximates the latticeparameter of the material surface of the semiconductor-based substrate,and then a film of the alkaline earth oxide is grown upon the materialsurface wherein the alkaline earth oxide film is at least one cell unitin thickness. An ABO₃ material, such as a perovskite, is subsequentlyselected which either has a lattice parameter closely approximating thelattice parameter of the semiconductor surface or the quotient of thelattice parameter of the semiconductor surface divided by the squareroot of 2.0. The ABO₃ material has a crystalline form comprised of twometal oxide planes wherein the metal oxide of one of the two metal oxideplanes is comprised of BO₂ so that the element B of the BO₂ planeprovides a small cation in the crystalline structure of the ABO₃material and wherein the metal oxide of the other of the two metal oxideplanes includes another metal which provides a large cation in thecrystalline structure of the ABO₃ material. A single plane of AO isgrown upon the alkaline earth oxide film wherein the AO of the singleplane is epitaxial and fully commensurate with the semiconductorsubstrate, and then a single plane comprised of the other of the twometal oxide planes (i.e. the oxide BO₂) of the perovskite crystallinestructure of the ABO₃ material is grown upon the AO plane so that themetal oxide of the other of the two metal oxide planes is epitaxial andfully commensurate with the AO plane and wherein the orientation of thegrown ABO₃ material is either oriented cube on cube with respect to thesurface of the substrate or is rotated 45° with respect to the surfaceof the substrate so that (001) perovskite is parallel to (001)semiconductor surface and 100! perovskite is parallel to 110!semiconductor surface.

In one aspect of the invention, the structure is formed by the processof the invention, and in another aspect of the invention, the structureis in the form of a ferroelectric field-effect (FFET) transistorincluding a base substrate of silicon, a source electrode, a drainelectrode, a gate electrode, and a gate dielectric interposed betweenthe silicon and the gate electrode. In the FFET, the improvement ischaracterized in that the gate dielectric includes an epitaxial thinfilm layer of a perovskite oxide interposed between the silicon and theremainder of the gate dielectric. The construction process used to buildup the FFET avoids any tendency for undesirable silicon dioxide (SiO₂)to form at the interface of the silicon and the gate dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a silicon wafer upon which a singlecrystal film of the perovskite BaTiO₃ can be grown in accordance withthe method of the present invention.

FIG. 2 is an exploded perspective view of a structure within which aperovskite film is grown upon a silicon substrate and illustratingschematically the successive layers of constituents comprising thestructure.

FIG. 3 is a schematic perspective view of a fragment of the ultra highvacuum equipment with which steps of the process of the presentinvention can be performed.

FIG. 4 is a plan view illustrating schematically the orientation of thelattice structures of adjacent constituent layers of the FIG. 2structure.

FIG. 5 is a schematic cross sectional view of a fragment of aferroelectric field effect transistor (FFET) utilizing a perovskite thinfilm as a gate dielectric.

FIG. 6 is a schematic cross sectional view of a fragment of a capacitorutilizing a perovskite layer juxtaposed with a layer of silicon.

FIG. 7 is a TEM photograph of a BaTiO₃ /CaTiO₃ /BaSrO/Si structure inaccordance with an embodiment of the structure of the present invention.

FIG. 8 is a graph wherein capacitance is plotted against gate voltage intests performed upon an embodiment of a structure in accordance of thepresent invention.

FIG. 9 is a graph wherein leakage current is plotted against gatevoltage in tests performed upon the structure embodiment tested in FIG.8.

FIG. 10 is a graph depicting the test results involving apolarization-induced shift of the capacitance voltage characteristics ofan embodiment of a structure in accordance with the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention truncates silicon with a stable perovskitestructure permitting growth of a thin-film ferroelectric material onsilicon as a monolithic structure. It is a member of our general seriesof commensurate structures designated as (AO)_(n) (A'BO)_(m) in which nand m are the integer repeats of single plane commensurate oxide layers.If n=1, then the perovskite is grown directly as ABO₃ from the silicidetruncation of silicon beginning at the AO plane. If n>1, theface-centered NaCl-type structure is grown at the interface thentruncated with the BO₂ plane to transistion to the perovskite structure.

With reference to FIG. 1, there is illustrated a wafer or substrate 20having a surface 22 upon which a single crystal film of a materialhaving the general formula ABO₃, such as a perovskite (e.g. BaTiO₃), canbe grown to produce a monolithic structure embodying features of thestructure of the present invention. The substrate 20 is preferably of asemiconductor-based material such as silicon or a silicon-germaniumalloy, but the substrate may be selected from group consisting of GroupIV, Group III-V and II-VI semiconductors.

The crystalline form of the ABO₃ material includes a first singleconstituent oxide plane having the general formula AO and a secondconstituent oxide plane having the general formula BO₂. While theelement 0 of the formula ABO₃ is understood to be oxygen, the element Amay be a material found in Group IA, IIA or IVB of the periodic table ofthe elements, while the element B may be a material found in Group III,IVA or VA of the periodic table. When the metal elements A and B ofcrystalline form of the ABO₃ material are compared to one another, theelement A provides a large cation in the crystalline structure of theABO₃ material, and the element B provides a small cation in thecrystalline structure of the ABO₃ material.

Briefly, during the build up of the desired ABO₃ upon the substratesurface 22, a first epitaxial and fully commensurate film of an alkalineearth oxide (having the general formula AO and a sodium chloride-typecrystal lattice structure) is grown upon the substrate surface 22, asecond film (of a desired A'BO₃ material) is grown upon the first film,and a third film (of the desired A'BO₃ material) is grown upon thesecond film. The element A' of the A'BO₃ material may, where AO is asingle atomic layer, be the same element A of the alkaline earth oxideAO having the sodium chloride-type lattice structure but may, in otherinstances, be an element other than the element A. Therefore, theformula A'BO₃ appropriately is designated A'BO₃ to differentiate, wherein the case of a single atomic layer of AO, the element A of thealkaline earth oxide AO is different from the element A' of theconstituent oxide A'O of the A'BO₃ material. It will therefore beunderstood that in the interests of the present invention, the elementA' of the formula A'BO₃ material can consist of, but is not limited to,the element A of the alkaline earth oxide AO.

As the aforedescribed structure is grown, the orientation of thecrystalline form of the second film being grown is either grown cube oncube or is rotated 45° with respect to the orientation e.g. (001)truncation! of the first (alkaline earth oxide) film to facilitate theepitaxial and fully commensurate build-up of the third film upon thesecond film. Therefore and as will be apparent herein, the first(alkaline earth oxide) film serves as a template upon which the secondfilm (of A'BO₃ material) is grown, and the second film serves as atemplate upon which the desired third film (of A'BO₃ material) is grown.

By way of example and not as limitation, the specific monolithicstructure described herein involves a substrate 20 ofsemiconductor-based material comprised of silicon, an alkaline earthoxide (AO) film comprised of Ba₀.725 Sr₀.275 O, and an A'BO₃ materialcomprised of the perovskite CaTiO₃ or, more specifically, perovskites ofthe CaTiO₃ class. While the material of the substrate 20 is generallycharacterized by a face-centered-cubic (fcc) lattice structure, such assilicon and silicon-germanium alloys, the alkaline earth oxide (AO)material includes a sodium chloride-type lattice structure, and theperovskites of the CaTiO₃ class are generally characterized by a simplecubic lattice structure. It will be understood, however, that theprinciples of the present invention can be used to build up thin-filmsof other A'BO₃ materials upon a substrate of another semiconductor-basedmaterial, such as a silicon-germanium alloy.

The techniques described herein to construct the desired resultantmonolithic structure are molecular beam epitaxy (MBE) techniques. Itwill be understood, however, that the described MBE techniques areintended for the purpose of illustration and not as limitation. Forexample, alternative methods, such as chemical vapor deposition (CVD)and metal organic chemical vapor deposition (MOCVD), can be employed.Accordingly, the principles of the present invention can be variouslyapplied.

As is described herein in accordance with an embodiment of the method ofthe present invention and with reference to FIG. 2, steps are taken tocover the surface 22 with a thin alkaline earth oxide film 24 of Ba₀.725Sr₀.275 O, then to cover the film 24 with a thin perovskite (template)film 26 of Ca₀.64 Sr₀.36 TiO₃, and then to cover the film 26 with adesired (multi-stratum) perovskite film 28 of a perovskite of the BaTiO₃class to provide a resultant structure 32. Each of the alkaline earthoxide film 24 and the template film 26 and an appreciable portion of theperovskite film 28 are constructed in somewhat of a singleplane-layer-by-single plane-layer fashion, described herein, to ensurecommensurate periodicity throughout the build up of each film andwherein the layer-construction processes take into account thecrystalline form of the material out of which the film is desired to beconstructed. Furthermore, the film-growing processes described hereintake advantage of the lattice matching that exists at the interface ofadjacent films of the structure 32. To this end, the lattice structuresat the interface of adjacent films have parameters which are matched sothat the likelihood of any appreciable lattice strain at the film/filminterface is significantly reduced. Moreover, the growth processdescribed herein avoids any propensity for silica (SiO₂) to form as anamorphous component of the interface template structure.

Unlike the face-centered-cubic (fcc) crystalline lattice structure ofthe semiconductor-based substrate 22, the crystalline lattice form ofperovskite is a simple cubic structure and its crystalline (i.e. cube)form includes a plane of a Group IVA element oxide, i.e. an oxide of agroup consisting of TiO₂, ZrO₂ and HfO₂, and another plane of adifferent metal oxide. For example and as discussed in U.S. Pat. No.5,450,812, having the same inventors as the instant application and thedisclosure of which is incorporated herein by reference, the crystallinelattice structure of the perovskite BaTiO₃ includes a plane of TiO₂ anda plane of BaO. Similarly, the crystalline form of the perovskite SrTiO₂includes a plane of TiO₂ and a plane of SrO.

At the outset of a process performed in accordance with the method ofthe present invention, the surface 22 of the silicon substrate 20 iscleaned to atomic cleanliness so that only silicon atoms are present atthe surface 22. To this end, the surface 22 is cleaned by a processcommonly referred to as a Modified RCA technique. The Modified RCAtechnique is a well-known process involving the chemical production ofan oxide at a silicon surface being cleaned and subsequently placing thesurface in a high vacuum environment and raising the temperature of thesurface to sublime the oxide off of the surface. (This samesurface-cleaning procedure is followed if the substrate 20 is comprisedof a silicon-germanium alloy.)

The layers of the desired structure 32 are built up in this example bymolecular beam epitaxy (MBE), electron beam evaporation techniques andwith MBE equipment. The MBE equipment includes an ultra high vacuum(UHV) growth/characterization facility, a fragment of which is indicated40 in FIG. 3. The facility 40 includes a container 42 having an innerchamber within which the substrate 20 is positioned so that its surface22 faces downwardly, and a plurality of canisters 44, 46, 48 and 50 areprovided within the base of the container 42 for providing a vaporsource of metals desired to be added to the substrate surface 22 duringthe formation of the structure 32. In this connection, each canister 44,46, 48 and 50 is adapted to hold a crucible containing a desired metaland contains heating elements for vaporizing the metal. An opening isprovided in the top of each canister, and a shutter is associated withthe canister opening for movement between a closed condition at whichthe interior of the container is closed and thereby isolated from thesubstrate surface 22 and an opened condition at which the contents ofthe container, i.e. the metal vapors, are exposed to the substratesurface 22.

In the depicted facility 40, an amount of the metal barium (Ba) ispositioned within the canister 44, an amount of strontium (Sr) ispositioned within the canister 46, an amount of calcium (Ca) ispositioned within the canister 48, and an amount of titanium (Ti) ispositioned within the canister 50. In addition, an oxygen source 52 isconnected to the chamber so that by opening and closing a valveassociated with the source 52, oxygen can be delivered to or shut offfrom the chamber. The opening and closing of each canister shutter andthe oxygen source valve is accurately controlled by a computercontroller (not shown).

Another feature of the facility 40 is that a closable substrate shutteris disposed immediately below the downwardly-directed face of thesubstrate surface 22 for isolating, when desired, the substrate surface22 from exposure to the metal vapors from the canisters or the oxygenfrom the oxygen source 52 while the internal pressure of the facilitychamber is raised with the oxygen from the source 52. The substrateshutter is closed during one step of the present process as will beapparent herein.

The vacuum drawn in the UHV facility 40 to complete the Modified RCAcleaning technique upon the substrate 20 is between about 10⁻⁹ and 10⁻¹⁰torr, and the substrate 20 is heated to raise the substrate temperatureto a temperature sufficient to drive the oxides off of the surface 22.In practice, such a temperature may be between about 850° and 1050° C.,and the desired surface cleanliness may be confirmed in-situ during thesubstrate heating operation by Reflection High Energy ElectronDiffraction (RHEED) techniques. For present purposes, the siliconsubstrate 20 reaches atomic cleanliness upon the development of 2×1Si(100) at the surface 22 as evidenced by RHEED analysis.

Upon reaching the desired atomic cleanliness and to initiate the growthof the first film 24 of the alkaline earth oxide, a mixture of apredetermined amount of Barium (Ba) metal and a predetermined amount ofStrontium (Sr) metal is deposited upon the substrate surface 22 so thata fraction, e.g. about one-fourth, of a monolayer of the mixture coversthe substrate surface 22. In other words, the Ba and Sr metal mixture isdeposited upon the substrate surface 22 until about one atom of themixture overlies the silicon surface 22 for every four atomic sites ofSi. To this end, Ba vapor and Sr vapor is created in the correspondingcanisters and the corresponding canister shutters are opened to exposethe clean substrate surface 22 to the Ba and Sr mixture.

The ratio of Ba to Sr in the Ba/Sr vapor mixture is selected with regardto the lattice parameter of the silicon structure (or, in thealternative, the silicon-germanium structure) of the substrate surface22. In particular, the lattice parameter of the silicon structure isknown to be 0.543 nm, and the lattice parameter of the structure of aBa_(x) Sr_(1-x) O compound (formed upon the substrate surface 22 in amanner described herein) is selected to closely match that of thesilicon structure so that when epitaxially covering the silicon surface22, no appreciable strain exists at the Si/Ba_(x) Sr_(1-x) O interface.In this connection, it is also known that the lattice parameter ofBa_(x) Sr_(1-x) O varies substantially linearly as the ratio of Sr to Bais increased in this compound from 0.0% to 100%. Thus, when the variable"x" in this compound equals 1.0, the lattice parameter of the compoundis 0.554 nm (corresponding with the lattice parameter of pure BaO), andwhen the variable "x" in the compound equals 0.0, the lattice parameterof the compound equals 0.514 nm (corresponding with the latticeparameter of pure SrO).

In the depicted example, the ratio of Ba to Sr in the Ba_(x) Sr_(1-x) Ocompound is selected to provide a lattice parameter of the Ba_(x)Sr_(1-x) O compound which exactly matches the lattice parameter ofsilicon or, in other words, is selected to provide the Ba_(x) Sr_(1-x) Ocompound with a lattice parameter of 0.543 nm. To this end, the variable"x" in the this compound equals 0.725 so that the proportion of BaO toSrO in the oxide compound eventually formed upon the substrate surface22 is 0.725 to 0.275.

In an alternative example in which the substrate 22 is comprised ofsilicon-germanium (Si_(y) Ge_(1-y)), the ratio of Ba to Sr in the Ba_(x)Sr_(1-x) O compound is selected to provide a lattice parameter of theBa_(x) Sr_(1-x) O compound which exactly matches the lattice parameterof Si_(y) Ge_(1-y). If, for example, the substrate 22 was comprised ofSi₀.80 Ge₀.20 which has a lattice parameter of 0.548 nm, the variable"x" in the aforementioned Ba_(x) Sr_(1-x) O compound is selected toequal 0.85 so that the proportion of BaO to SrO in the oxide compoundeventually formed upon the substrate surface 22 is 0.85 to 0.15 toprovide the Ba_(x) Sr_(1-x) O compound with a lattice parameter of 0.548nm.

For a more detailed description of the lattice matching between adjacentfilms for the purpose of reducing lattice strain at a film/filminterface of an epitaxial layup of films, reference can be had to U.S.Pat. No. 5,482,003, having the same inventors as the instant applicationand the disclosure of which is incorporated herein by reference.

Accordingly and with regard again to the exemplary substrate 22comprised of pure silicon, in the process step described herein in whichthe Ba and Sr metals are deposited upon a substrate of silicon so as toform a submonolayer thereon involves the exposure of the substratesurface 22 to a mixture of Ba and Sr vapors wherein the ratio of Ba toSr in the mixture is 0.725 to 0.275. Such exposure can be effected withthe facility 40 by either of two methods. One method involves theproduction of a flux vapor of Ba and a flux vapor of Sr from thecanisters 44 and 46 containing Ba and Sr, respectively, so that thecombined vapor fluxes emitted from the canisters provide the desired,i.e. target, ratio of Ba to Sr in the Ba/Sr vapor mixture. The othermethod involves the control of the amount of time that the shutters ofthe Ba and Sr-containing canisters are opened so that the appropriateamount of Ba and Sr vapors are emitted from the corresponding canistersand become mixed in the facility 40. In either event, the techniquesused to produce a mixture of metal vapors in the facility 40 wherein thevapor mixture contains a desired ratio of one metal vapor to anothermetal vapor involve techniques which are known and common to MBE so thatthe desired Ba to Sr in a mixture of Ba and Sr vapors can be achieved inthe facility with a high degree of accuracy.

Upon completion of the deposition of the desired fraction of themonolayer of Ba and Sr atoms upon the substrate surface 22, thesubstrate 20 is cooled to between about room temperature and 150° C.while the high vacuum environment is maintained about the substrate 20,and the remainder of one monolayer of Ba and Sr is then deposited uponthe substrate surface. To this end, the shutters of the canisters of Baand Sr can be opened for an appropriate period of time sufficient forthe desired mixture of Ba and Sr vapor (wherein the ratio of Ba to Sr inthe vapor mixture is 0.725 to 0.275) is exposed to the substrate. Bycooling the substrate 20 to the lower temperature, i.e. between aboutroom temperature and 150° C., the attachment of Ba and Sr atoms to thesubstrate surface is promoted because the added Ba and Sr atoms remainin a metallic state and do not form silicide at or below these lowertemperatures.

As has been addressed in earlier U.S. Pat. No. 5,225,031, having thesame inventors as the instant application and the disclosure of which isincorporated herein by reference, the purpose for developing themonolayer of Ba and Sr atoms at the Ba/Sr interface is to form a stabletemplate surface upon which a subsequent epitaxial layer of Ba₀.725Sr₀.275 O is grown. Thus, with the stable monolayer of Ba₀.725 Sr₀.275 Oformed upon the Si surface, Ba₀.725 Sr₀.275 O can be grown epitaxiallyupon the silicon in such a manner as to avoid the formation of amorphoussilica. To this end, the substrate shutter is closed to prevent exposureof the substrate surface 20 to the facility chamber contents, and thepressure of the chamber is raised to about 1 to 5×10⁻⁶ torr of oxygenwhile maintaining Ba and Sr vapor source operations that would be neededto deposit Ba and Sr metal upon the substrate surface at a predeterminedrate and in the desired, or target, proportions of Ba to Sr. Uponreaching the target oxygen pressure, e.g. 1×10⁻⁶ torr, the substrateshutter is opened to expose the Ba and Sr-coated surface of thesubstrate to oxygen and additional Ba and Sr atoms. Upon such exposure,Ba₀.725 Sr₀.275 O begins to grow epitaxially upon the Ba and Sr-coatedsurface.

By appropriately opening and closing off the exposure of the substratesurface to the Ba and Sr metals and oxygen by cyclically exposing thesubstrate surface to the Ba and Sr metals and oxygen, Ba₀.725 Sr₀.275 Ois grown upon the substrate surface one atomic layer at a time. Such agrowth pattern is continued until the monolayers of Ba₀.725 Sr₀.275 Odevelop sufficient stability to prevent the formation of an amorphoussilicate. It has been found that such stability is achieved upon theformation of a Ba₀.725 Sr₀.275 O thickness of about 1.0 nm (equivalentto about two cell units high), and it is at this thickness of two cellunits that the growth of the film 24 is halted and the growth of thesubsequent film 26 is initiated.

In other words, upon formation of the stable film 24 (of two cell unitsin thickness) of Ba₀.725 Sr₀.275 O upon the substrate surface 22, stepsare taken to form the desired template film 26 of Ca₀.64 Sr₀.36 TiO₃upon the film 24. Whereas the ratio of Ba to Sr in the film 24 of Ba_(x)Sr_(1-x) O is selected for its lattice match to that of the underlyingsilicon of the surface 22, the ratio of Ca to Sr in the film 26 ofCa_(x) Sr_(1-x) TiO₃ is selected for its lattice match to that of theunderlying film 24 of Ba₀.725 Sr₀.275 O. However, whereas the film 24 isgrown epitaxially upon and is fully commensurate with the siliconsurface 22 so that its lattice orientation matches that of the siliconsurface 22, the crystalline form of the Ca₀.64 Sr₀.36 TiO₃ film 26(which is also epitaxial and fully commensurate with the underlying film24) has an orientation which is rotated 45° with respect to theorientation of the crystalline form of the underlying Ba₀.725 Sr₀.275 Ofilm 24. As will be apparent herein, the build-up of the film 26 uponthe film 24 effects a change in the lattice structure of theconstruction from fcc (i.e. of the underlying semiconductor-basedmaterial) to the simple cubic lattice structure of a perovskite (i.e.Ca₀.64 Sr₀.36 TiO₃) while the perovskite build-up is fully commensuratewith the underlying semiconductor-based material, and this build-upprocess is advantageous in this respect.

In this connection, CaTiO₃ and SrTiO₃ are mutually soluble with oneanother and each has a cubic phase with a continuously variable latticeparameter from 0.380 nm for CaTiO₃ to 0.391 nm for SrTiO₃. With this inmind, the crystalline structure of the compound Ca_(x) Sr_(1-x) TiO₃wherein x=0.64 yields a lattice parameter of 0.384 nm, and this latticeparameter matches, with a 45° rotation, the 110! spacing of silicon(0.384 nm). Such a match of the Ca₀.64 Sr₀.36 TiO₃ lattice structureatop the Ba₀.725 Sr₀.275 O lattice structure is depicted in the planview of FIG. 4 wherein the Ba₀.725 Sr₀.275 O lattice structure (having alattice parameter of 0.543 nm) is depicted in solid lines in FIG. 4 andthe Ca₀.64 Sr₀.36 TiO₃ lattice structure (having a lattice parameter of0.384 nm) is depicted in phantom in FIG. 4. It follows that the desiredlattice parameter of the Ca₀.64 Sr_(1-x) TiO₃ crystal is the quotient ofthe lattice parameter of the underlying Ba₀.725 Sr₀.275 O crystal (0.543nm) divided by the square root of 2.0 (i.e. approximately 1.414).

The above-discussed 45° rotation of the orientation of the latticestructure grown atop another lattice structure has been proven bygrowing a sample of Ca_(x) Sr_(1-x) TiO₃ on BaSrO/Si in a growthsequence: TiO₂ /CaSrO/TiO₂ /CaSrO/ . . . Reflection high energy electrondiffraction (RHEED) from the (001) surfaces of the initial Si comparedwith the (001) face of CaTiO₃ show that epitaxy does develop with theexpected 45° rotation so that (001) CaTiO₃ is parallel to (001) siliconand 100! CaTiO₃ is parallel to 110! silicon. The alloyed and latticematched Ca_(x) Sr_(1-x) TiO₃ thin films are stable after growth as thinas 3 unit cells (less than 1.2 nm).

To grow the desired template perovskite film 26 of Ca₀.64 Sr₀.36 TiO₃upon the film 26, steps are taken which correspond to those set forth inthe earlier-referenced U.S. Pat. No. 5,450,812. Briefly and keeping inmind that the crystalline form of the perovskite structure Ca₀.64 Sr₀.36TiO₃ includes a plane of TiO₂ and a plane of Ca₀.64 Sr₀.36 O, singleplanes of TiO₂ and Ca₀.64 Sr₀.36 O are grown in an alternating fashion(starting with a single plane of TiO₂) upon the BaO₀.725 Sr₀.275 O film24 until the desired thickness of the film 26 is obtained.

In preparation of the growth of an initial TiO₂ plane of the film 26,the pressure in the UHV chamber is adjusted to (or maintained) betweenabout 2-5×10⁻⁷ torr. The desired plane of TiO₂ is then built upon theMgO surface by conventional MBE techniques while the internal pressureof the facility 40 is maintained between about 2-5×10⁻⁷ torr. Forexample, Ti metal vapor could initially be deposited upon the Ba₀.725Sr₀.275 O surface and then oxygen from the source 40 could be releasedover the surface so that the desired layer of TiO₂ is formed at theBa₀.725 Sr₀.275 O surface. Alternatively, the Ba₀.725 Sr₀.275 O surfacecould be simultaneously exposed to Ti vapor and oxygen, in controlledamounts, so that TiO₂ forms and then accumulates on the Ba₀.725 Sr₀.275O surface.

During either of the aforementioned deposition processes involving theTiO₂ layer, careful control of the MBE operation is maintained to ensurethat no more than a single plane-layer, i.e. one plane, of TiO₂ isdeposited upon the Ba₀.725 Sr₀.275 O surface. The bulk form of thecompound TiO₂, as characterized by the ordered surface structure formedin this step, has a non-equilibrium structure and is not found innature, and there exists a tendency for the formed TiO₂ to accumulateinto clusters if the Ba₀.725 Sr₀.275 O surface is exposed to a greateramount of TiO₂ than is needed to comprise a single plane of TiO₂. Ofcourse, if such clusters develop, the TiO₂ layer looses its order, andthe ability to grow ordered layers upon the TiO₂ layer is destroyed.Thus, careful control must be maintained over the deposition of Ti vaporand the release of oxygen from the source 40 so that a single plane, andonly a single plane, of TiO₂ accumulates at ordered sites upon theBa₀.725 Sr₀.275 O surface (i.e. directly contacts and is fullycommensurate with the Ba₀.725 Sr₀.275 O surface.

Following the development of the desired initial (single plane) layer ofTiO₂ upon the Ba₀.725 Sr₀.275 O surface, a (single plane) layer ofCa₀.64 Sr₀.36 O which comprises the other plane of the perovskite Ca₀.64Sr₀.36 TiO₃ is grown upon the TiO₂ layer. To this end, conventional MBEtechniques are used to grow the desired Ca₀.64 Sr₀.36 O layerepitaxially upon and fully commensurate with the formed TiO₂ layer. Forexample, the metal vapors Ca and Sr may be initially deposited upon theTiO₂ surface in the desired proportions, i.e. 0.64 to 0.36, and then theoxygen may be subsequently released into the chamber so that the Ca₀.064Sr₀.36 O forms upon the TiO₂ surface. Alternatively, the TiO₂ layercould be simultaneously exposed to Ca and Sr vapors and oxygen so thatCa₀.64 Sr₀.36 O accumulates on the TiO₂ layer. In either event, carefulcontrol should be maintained over the deposition operation here so thatone plane, and no more than one plane, of the desired layer of Ca₀.64Sr₀.36 O is developed at this stage upon the TiO₂ layer.

Upon formation of the desired plane of Ca₀.64 Sr₀.36 O, a second planeof TiO₂ is grown upon the Ca₀.64 Sr₀.36 O plane in accordance with theaforedescribed techniques used to grow TiO₂ onto the Ba₀.725 Sr₀.275 Osurface. Then, upon formation of the desired second plane of TiO₂, asecond plane of Ca₀.64 SrO₀.36 O is grown upon the second plane of TiO₂.

Thereafter, single plane-layers of TiO₂ and Ca₀.64 Sr₀.36 O are formedin an alternating fashion until at least about three cell units of thedesired CaSrTiO₃ perovskite are grown upon the Ba₀.725 Sr₀.275 Osurface. It has been found that the alloyed and latticed matched Ca₀.64Sr₀.36 TiO₃ film is stable after growth as thin as three unit cells (1.2nm) is obtained. Accordingly, the growth of the film 26 is halted uponthe obtaining of a thickness of the film 26 of three unit cells.

Once the desired template film 26 of Ca₀.64 Sr₀.36 TiO₃ is formed, stepsare taken to grow the desired perovskite film 28 upon the film 26. Inthe embodiment of the method described herein and as mentioned earlier,the perovskite of the film 28 is BaTiO₃, and steps can be taken to growthe BaTiO₃ directly upon the film 26, but as will be discussed herein,there exists alternative schemes by which a film of BaTiO₃ is ultimatelyobtained.

To grow BaTiO₃ directly upon the template film 26, steps are taken togrow BaTiO₃ in a single plane-layer-by-single plane-layer, i.e. aconstituent plane-by-constituent plane, fashion until a critical cellunit height is achieved or, in other words, until lattice strain ceasesto appear at the surface of the layup of planes. In this connection andkeeping in mind that the cubic crystalline form of BaTiO₃ is comprisedof a plane of TiO₂ and a plane of the metal oxide BaO, an initial filmlayer comprised of a single plane of TiO₂ is grown epitaxially upon thesurface of the Ca₀.64 Sr₀.36 TiO₃ film 26. As discussed above inconnection with the growth of a TiO₂ plane of the film 24 and whilemaintaining the internal pressure of the facility 40 between about2-5×10⁻⁷ torr, Ti metal vapor could initially be deposited upon theCa₀.64 Sr₀.36 TiO₃ surface and then oxygen from the source 40 could bereleased over the surface so that the desired layer of TiO₂ is formedthereon. Alternatively, the Ca₀.64 Sr₀.36 TiO₃ surface could besimultaneously exposed to Ti vapor and oxygen, in controlled amounts, sothat TiO₂ forms and then accumulates on the Ca₀.64 Sr₀.36 TiO₃ surface.As has been described in connection with the aforementioned depositionprocesses involving a single plane-layer of TiO₂, careful control of theMBE operation is maintained to ensure that no more than one plane ofTiO₂ is deposited directly upon the Ca₀.64 Sr₀.36 TiO₃ surface.

Following the development of the desired layer of TiO₂ upon the Ca₀.64Sr₀.36 TiO₃ surface, a (single plane) layer of BaO which comprises theother plane of the crystalline structure of the perovskite BaTiO₃ isgrown upon the initial TiO₂ plane. As is the case with the formation ofthe plane of metal oxide Ca₀.64 Sr₀.36 TiO₃ of the film 26, the metaloxide BaO can be grown directly upon the TiO₂ plane by conventional MBEtechniques. For example, the metal vapor Ba may be initially depositedupon the TiO₂ surface, and then the oxygen may be subsequently releasedinto the chamber so that the metal oxide BaO forms upon the TiO₂surface. Alternatively, the TiO₂ layer could be simultaneously exposedto metal vapor and oxygen so that the metal oxide BaO accumulates on theTiO₂ layer. Again, careful control should be maintained over thedeposition operation here so that one plane, and no more than one plane,of the desired metal oxide BaO is developed at this stage upon the TiO₂layer and so that the pattern of metal oxide deposited upon the TiO₂layer is epitaxial and fully commensurate with the TiO₂ of the TiO₂layer.

Upon formation of the desired plane of metal oxide BaO, another plane ofTiO₂ is grown upon the metal oxide plane in accordance with theaforedescribed techniques used to grow TiO₂ upon the Ca₀.64 Sr₀.36 TiO₃surface of the film 24. Upon formation of the desired another plane ofTiO₂, another plane of the metal oxide BaO is grown upon the secondplane of TiO₂.

Thereafter, single plane-layers of TiO₂ and BaO are formed in analternating fashion until a critical thickness of the desired perovskiteBaTiO₃, corresponding in this instance to a cell unit height of at leastabout twelve cell units, is grown upon the film 24. In other words,dislocations which may develop within the formed layers nucleate so asto provide internal strain relief within about the first twelve cellunits so that lattice strain does not appear at the surface of the layupof planes. Thus, the surface defined by the twelfth cell unit is orderedand substantially free of strain.

Once the strain-free surface of perovskite is formed, steps can be takento grow additional layers of the perovskite BaTiO₃ upon the build up ofcell units. In this connection, subsequent growth of the perovskite uponits strain-free bulk form is homoepitaxial, rather than heteroepitaxialso that the characteristics of the interface between adjacent layers ofTiO₂ and the metal oxide BaO are not likely to present problems duringgrowth. Thus, the perovskite can be built upon itself in layers whichare each one cell unit in height after the initial twelve cell units ofthe perovskite are formed. To this end, the perovskite BaTiO₃ is grownsingle cell-layer-by-single cell-layer upon the strain-free surface byconventional MBE techniques so that each layer grown during this stageis one cell unit high. For example, the strain-free surface ofperovskite may initially be exposed to Ti and the metal Ba vapors andthen to oxygen so that the perovskite forms upon the strain-freesurface. Alternatively, the strain-free surface can be exposedsimultaneously to the Ti and Ba vapors and oxygen so that the perovskiteforms and then settles upon the strain-free surface. Still further,known co-deposition techniques (e.g. other than MBE processes) can beemployed to grow the perovskite in this stage of the growth process. Ineither instance, careful control of the build up process is maintainedso that the build up of successive layers of the perovskite is effectedepitaxially.

As an alternative to growing BaTiO₃ directly upon the film 26 of Ca₀.64Sr₀.36 TiO₃, an intermediate perovskite film of Ba_(x) Sr_(1-x) TiO₃ canbe grown upon the film 26, wherein the variable "x" in the compositionis chosen so that the lattice parameter of the perovskite crystallinestructure is closer to that, i.e. 0.384 nm, of the underlying Ca₀.64Sr₀.36 TiO₃ perovskite film than is the lattice parameter of BaTiO₃(which is 0.4 nm). To this end, the variable "x" in the Ba_(x) Sr_(1-x)TiO₃ compound is chosen to be 0.725.

To grow Ba₀.72 Sr₀.36 TiO₃ upon the film 26 and keeping in mind that thecrystalline structure of this perovskite includes a single plane of TiO₂and a single plane of Ba₀.725 Sr₀.275 O, an initial film layercomprising a single plane of TiO₂ is grown epitaxially upon the surfaceof the Ca₀.64 Sr₀.36 TiO₃ film 26. The aforedescribed conventional MBEtechniques can be used to grow the initial film layer of TiO₂. Of courseand has been described in connection with the build up of the Ca₀.64Sr₀.36 TiO₃ and BaTiO₃ films, careful control of the MBE operation ismaintained during the build up of this initial single plane-layer ofTiO₂ to ensure that one plane, and no more than one plane, of TiO₂ isgrown upon the Ca₀.64 Sr₀.36 TiO₃ surface.

Subsequent to the build up of the initial TiO₂ plane, Ba₀.725 Sr₀.275 Owhich comprises the other plane of the perovskite Ba₀.725 Sr₀.275 TiO₃is formed upon the single plane layer of TiO₂. To this end, conventionalMBE techniques are used to grow the desired Ba₀.725 Sr₀.275 O layer uponthe formed TiO₂ layer. For example, the metal vapors Ba and Sr may beinitially deposited upon the TiO₂ surface in the appropriate ratio 0.725to 0.275, and then the oxygen may be subsequently released into thechamber so that the desired Ba₀.725 Sr₀.275 O forms upon the TiO₂surface. Alternatively, the TiO₂ layer could be simultaneously exposedto Ba and Sr vapors in the appropriate amounts and oxygen so that thedesired Ba₀.725 Sr₀.275 O accumulates on the TiO₂ layer. Again, carefulcontrol should be maintained over the deposition operation here so thatno more than one plane of the desired layer of Ba₀.725 Sr₀.275 O isdeveloped at this stage upon the TiO₂ layer and so that the pattern ofBa₀.725 Sr₀.275 O deposited upon the TiO₂ layer is epitaxial and fullycommensurate with the TiO₂ of the previously-grown initial TiO₂ plane.

Upon formation of the desired plane of Ba₀.725 Sr₀.275 O, a furtherplane of TiO₂ is grown upon the Ba₀.725 Sr₀.275 O plane in accordancewith the aforedescribed techniques used to grow TiO₂ onto the Ba₀.725Sr₀.275 O surface. Then, upon formation of the desired further plane ofTiO₂, a further plane of Ba₀.725 Sr₀.275 O is grown upon the furtherplane of TiO₂. Thereafter, single plane layers of TiO₂ and Ba₀.725Sr₀.275 O are grown in an alternating fashion atop one another until acell height is reached at which no lattice strain appears in thelast-grown layer, or plane, of Ba₀.725 Sr₀.275 O. In other words, anylattice strain which may exist at the Ba₀.725 Sr₀.275 O/Ca₀.64 Sr₀.36TiO₃ interface is not as apparent as the surface of subsequently-formedlayers of TiO₂ and Ba₀.725 Sr₀.275 O. Along these lines, it is believedthat no such strain will appear following a build up of about four cellunits of the Ba₀.725 Sr₀.275 TiO₃ perovskite structure upon the Ca₀.64Sr₀.36 TiO₃ film 26.

Following the growth of the Ba₀.725 Sr₀.275 TiO₃ perovskite structure,BaTiO₃ can be grown upon the Ba₀.725 Sr₀.275 TiO₃ surface one cell unitlayer-at-a-time by conventional MBE techniques so that each layerconstructed at a stage of the build up process is one cell unit high.For example, the Ba₀.725 Sr₀.275 TiO₃ surface may be initially beexposed to Ti and Ba vapors and then to oxygen so that BaTiO₃ perovskiteforms upon the strain-free surface. Alternatively, the Ba₀.725 Sr₀.275TiO₃ surface can be exposed simultaneously to the Ti and Ba vapors andoxygen so that the BaTiO₃ perovskite forms and then settles upon theBa₀.725 Sr₀.275 TiO₃ surface. In either instance, careful control of theMBE process is maintained so that the build up of successive layers ofthe perovskite is effected epitaxially. The growth of BaTiO₃ iscontinued until the desired thickness of BaTiO₃ is obtained.

To illustrate the ordered arrangement of a multiplane structureembodying the desirable features which can be achieved with the processdescribed above, there is shown in FIG. 7 a transmission electronmicrograph (TEM) of a cross section of a BaTiO₃ /CaTiO₃ /BaSrO/Sistructure in accordance with an embodiment of the structure of thepresent invention. The layer of BaSrO which directly contacts and isfully commensurate with the underlying substrate of silicon is fouratoms thick, and the BaSrO interface with silicon is atomically sharpwith no evidence that amorphous silica is present at that interface. Bycomparison, the layer of CaTiO₃ which directly contacts and is fullycommensurate with the underlying layer of BaSrO is eight atoms thick. Alayer of the perovskite BaTiO₃ directly contacts and is fullycommensurate with the underlying layer of CaTiO₃. It can be seen withinthis FIG. 7 TEM that the atoms of the various layers are highly orderedand uniform and that the planes comprising the various layers of theFIG. 7 structure are substantially defect-free.

It will be understood that although the aforedescribed structure hasbeen described as involving a build up of BaTiO₃ upon asemiconductor-based substrate, other perovskites can be constructed inaccordance with the broader aspects of the invention. Such perovskitesinclude those in the BaTiO₃ class such as CaTiO₃, PbTiO₃, PbLaTiO₃,Pb(Zr Ti)O₃, (PbLa)(ZrTi)O₃, SrTiO₃, KNbO₃, KTaO₃, NaNbO₃, NaTaO₃,LiNbO₃, LiTaO₃, CaTiO₃, LaAlO₃, NaTaO₃ and YBCO.

Although the build up of the structure 32 has been described asinvolving the use of an intermediate template layer 26 of Ca_(x)Sr_(1-x) TiO₃ wherein x=0.64, we have found experimentially (i.e.verified through RHEED analysis) that in order to achieve the desiredcommensurate periodicity between sequentually-built single-plane layers,the ratio of Ca to Sr within the single plane layers of Ca_(x) Sr_(1-x)O may fall within a relatively broad range (e.g. wherein "x" may fallwithin the range of between 0.5 and 0.8). Accordingly, wherein x isdescribed within the layer 26 of Ca_(x) Sr_(1-x) TiO₃ in theaforedescribed structure as equal to 0.64, the value of "x" is notnecessarily so limited.

Ferroelectric Considerations

It is recognized in the art that ferroelectric materials, such asperovskites, can be advantageously used in solid state electricalcomponents if incorporated therein in a manner which takes appreciableadvantage of the ferroelectric and/or dielectric properties of thematerials. For example and with reference to FIG. 5, there is shown aferroelectric field effect transistor (FFET), indicated 70, including abase, or substrate 72 of Si and an overlayer 84 of the perovskiteBaTiO₃. The transistor 70 is also provided with a source electrode 78, adrain electrode 80, a gate electrode 82, and a gate dielectric 83. TheBaTiO₃ thin-film 84 (which comprises a portion of the gate dielectric83) is sandwiched between the epilayer 76 and the remainder of the gatedielectric 83 so as to be positioned adjacent the epilayer 76. Sinceferroelectric materials possess a permanent spontaneous electricpolarization (electric dipole moment per cubic centimeter) that can bereversed by an electric field, the ferroelectric dipoles can beswitched, or flipped, and the charge density and channel current can bemodulated. Thus, the transistor 70 can be turned ON or OFF by the actionof the ferroelectric polarization, and if used as a memory device, thetransistor 70 can be used to read the stored information (+ or -, or "1"or "0") without ever switching or resetting (hence no fatigue).

Similarly, there is schematically depicted in FIG. 6 a capacitor 90 fora dynamic random access memory (DRAM) circuit including a silicon layer92 and an oxide (dielectric) layer 94 which are in superposedrelationship and which are sandwiched between a gate 96 and a groundterminal 98. In use, an information-providing signal is collected fromthe capacitor 90 by measuring the current of the capacitor 90 during adischarge cycle. Therefore, the greater the dielectric constantexhibited by the oxide layer 94, the greater the charge-storage capacityof the capacitor 90. Since ferroelectric materials, such as perovskites,are known to be capable of exhibiting relative large dielectricconstants (e.g. at least 1000), a perovskite-including capacitor whichtakes appreciable advantage of the desirable dielectric properties ofthe perovskite would be advantageous.

Heretofore, however, in the case of each of the ferroelectric fieldeffect transistors and capacitors or inactive gate transistors whichincorporate a ferroelectric material, such as a perovskite, the devicesare incapable of taking appreciable advantage of the ferroelectricand/or dielectric properties of the ferroelectric materials. The FFETsconstructed to date have been unsatisfactory in performance, and thecapacitors and inactive gate transistors constructed to date have beentoo leaky and thus incapable of holding a charge for a lengthy period oftime. Factors which are responsible for the unsatisfactory performanceof FFETs or ferroelectric material-including capacitors or inactive gatetransistors include the impurities (e.g. amorphous nature) of thecrystalline structure of the material or the interface between theferroelectric material and the underlying silicon which interferes withthe flow of current within the device. For example, some interfacematerials employed in FFETs can screen and thereby trap charge thatcould otherwise contribute to the depleted or accumulated state of thecurrent-carrying channel of the device.

The aforedescribed process of the present invention can be used toincorporate a ferroelectric material, i.e. a perovskite, in a solidstate electrical component, such as a FFET and a capacitor for a RAM orDRAM circuit, during the construction of the component which enables thecomponent to take appreciable advantage of the ferroelectric and/ordielectric properties of the ferroelectric material during use. In otherwords, by building up a desired perovskite directly upon silicon withthe use of the template structure as described above, the crystallinequality of the resulting perovskite is high, and the interface betweenthe perovskite and the silicon is stable. Along these lines, the fewlayers of non-perovskite interface material which provide the templatestructure upon which the perovskite is constructed are commensurate withless than 1.0×10⁴ site fraction errors thereby achieving a monolithicinterface structure. Thus, interface trap densities of less than 10¹¹per square cm are achieved.

When applying the foregoing to FFET construction and with referenceagain to FIG. 5, the overlayer 84 of the perovskite BaTiO₃ is grown uponthe substrate 72 of Si in accordance with the process of the presentinvention to provide the FFET 70 with an overlayer 72 of highcrystalline quality and with a stable perovskite/silicon interface.Similarly, when applying the foregoing to capacitor construction andwith reference again to FIG. 6, the oxide layer 94 of the capacitor 90can be provided by the perovskite BaTiO₃ grown upon the silicon layer 92in accordance with the process of the present invention to provide thecapacitor with an oxide layer 94 of high crystalline quality and astable oxide/silicon interface structure. This construction, whensuitably modified as previously noted, is also applicable tosilicon-germanium-based devices.

Moreover, by exactly matching the lattice parameters of an overgrowingoxide with those of silicon, heteroepitaxy with a perovskite structurelike BaTiO₃ can be accomplished avoiding interfacial strain and therebyimproving the interfacial coherence and crystalline quality of thesilicon/ferroelectric thin-film structure. Furthermore, the long rangestructural coherence of single crystal BaTiO₃ thin films on siliconimprove the dielectric properties of thin film memory devices andsignificantly improve their fatigue life in read-write-restore cycles ofa conventional memory circuit which is normally limited by the formationand interaction of both line and planar defects in polycrystallinematerials presently used. Still further, the absence of internal grainboundaries, strain, and electrostatic field effects commonly associatedwith the grain boundaries will significantly extend the useful life of athin film ferroelectric memory structure.

To substantiate that an embodiment of a structure of the presentinvention does indeed possess the desirable qualities addressed above,there is provided in FIGS. 8-10 graphs of data collected from samplescomprised of a layer of BaTiO₃ constructed atop a silicon substrate inaccordance with an embodiment of the method of the present invention. Inparticular, FIG. 8 is a graph depicting the measured capacitance versusgate voltage of a layer of BaTiO₃ (0.280 nm in thickness) constructedupon silicon. The curve drawn through the plotted points characterizethat of a substance suitable for use as a capacitor (e.g. an MOScapacitor). Along the same lines, the FIG. 9 plot which shows theleakage current versus gate voltage of the material illustrates a lowleakage current (i.e. less than 10⁻⁹ amps per square centimeters at 3.0volts)--a quality indicting that the material (as a capacitor) will holda charge for an appreciable period of time. Furthermore, the curvesdepicted in FIG. 10 evidence a threshold voltage shift (of about 1.0volt) as a consequence of a polarization reversal in the ferroelectricgate oxide. Therefore, when the structure is used in conjunction with aFFET (such as the FFET 70 of FIG. 5), the polarization reversal switchesthe silicon and thus switches the device ON or OFF.

Still further, a sample capacitor construction (constructed inaccordance with the method of the present invention) including athin-film of BaTiO₃ (of 0.280 nm in thickness) grown onto an interfacethin-film of CaTiO₃ (of 0.40 nm in thickness) grown onto a siliconsubstrate has been found to possess the following MOS capacitorcharacteristics: The flat band voltage measured -1.027 volts; thethreshold voltage measured -0.29 volts; the Al/Si workfunction (volts)measured -0.95 volts; the interface charge (coul/cm²) measured 6.04×10⁻⁸coul/cm² ; and the trap density (1/cm²) measured 3.77×10¹¹. In addition,the resistivity-voltage, has been found to be 10¹³ ohm-cm, and theleakage current is less than 1×10⁻⁹ amps/cm² at 3 volts. The foregoingmeasurements were made with aluminum electrodes, 160 μm pads, p-doped10¹⁶ /cm².

It follows from the foregoing that a FFET and a process for constructingthe FFET has been described which improves upon conventional FETstructure. In particular, a monolithic structure and process has beendescribed which accommodates the lattice mismatch between silicon and aperovskite, such as BaTiO₃, if a single crystal of the perovskite is tobe grown upon silicon. After limiting the thickness of theinitially-grown alkaline earth oxide film to two unit cells (e.g.2×0.543 nm, or 1.068 nm), a unique transition is made to a perovskitestructure, CaTiO₃ (having a cubic lattice parameter of 0.380 nm) whichcan be alloyed with Sr to exactly lattice match silicon. Since CaTiO₃and SrTiO₃ are mutually soluble in each other and have a cubic phasewith continuously variable lattice parameter from 0.380 nm for CaTiO₃ to0.391 nm for SrTiO₃, the composition Ca_(x) Sr_(1-x) TiO₃ wherein x=0.64has a crystalline structure which, with a 45° rotation of itsorientation, lattice-matches the 110! spacing of silicon (0.384 nm).BaTiO₃ or SrTiO₃ are simple cubic perovskites and when grown epitaxiallyupon Ca_(x) Sr_(1-x) TiO₃ on BaSrO/Si as the active component in acomposite ferroelectric structure developed on silicon, are the centralelements of a thin film memory circuit.

Similarly, it follows from the foregoing that a ferroelectricmaterial-including capacitor or an inactive gate transistor and aprocess for constructing the device has been described which improvesupon conventional capacitor or inactive gate transistor. Whereas in aFFET, the ferroelectric material incorporated therein is used in aferroelectric state as a ferro-gated transistor, in applications such asa capacitor used in a DRAM circuit or an inactive gate transistor, theferroelectric material is used in a non-ferroelectric state, but as ahigh dielectric constant configuration for inactive gate transistors orcapacitors.

It will be understood that numerous modifications and substitutions canbe had to the aforedescribed embodiments without departing from thepresent invention. For example, while much of the foregoing discussionhas focused upon the ferroelectric qualities of a perovskite constructedon a semiconductor-based material, it will be understood by one skilledin the art that many comparable devices can be constructed in accordancewith the principles of this invention which possess other desirablecharacteristics. For example, comparable devices can be constructedwhich are piezoelectric in nature, pyroelectric in nature orelectro-optic in nature. Accordingly, the aforedescribed embodiments areintended for the purpose of illustration and not as limitation.

We claim:
 1. A monolithic crystalline structure comprising:asemiconductor substrate having a surface; and a film overlying thesurface of the substrate wherein the film consists of an ABO₃ materialhaving at least one AO constituent plane and at least one BO₂constituent plane wherein said ABO₃ material is either arranged in acube-on-cube relationship with the surface of the substrate or has alattice parameter which closely approximates the quotient of the latticeparameter of the surface of the substrate divided by the square root of2.0; and the film being arranged upon the surface of the substrate sothat a first single plane consisting of a single atomic layer of said AOconstituent of the ABO₃ material overlies and is commensurate with thesurface of the substrate and a second single plane consisting of asingle atomic layer of said BO₂ constituent of the ABO₃ materialoverlies and is commensurate with the first single plane of AO.
 2. Aprocess for growing a film onto the surface of a semiconductor-basedmaterial having a surface, the process comprising the steps of:a)providing an atomically-clean substrate of semiconductor-based materialhaving a surface; b) positioning the substrate within an oxygen-freeenvironment; c) selecting an alkaline earth oxide having a latticeparameter which closely matches the lattice parameter of the material ofthe semiconductor-based substrate; d) growing a film of the alkalineearth oxide upon the surface of the substrate wherein the alkaline earthoxide of the film being grown in this step d) is commensurate with thesurface of the semiconductor-based substrate; e) selecting an ABO₃material having a lattice parameter which closely approximates eitherthe lattice parameter of the substrate material or the quotient of thelattice parameter of the substrate material divided by the square rootof 2.0 and wherein the ABO₃ material has a crystalline form comprised oftwo metal oxide planes wherein the metal oxide of one of the two metaloxide planes is comprised of BO₂ so that the metal element B of the BO₂plane provides a small cation in the crystalline structure of the ABO₃material and wherein the metal oxide of the other of the two metal oxideplanes is comprised of AO so that the metal element B provides a largecation in the crystalline structure of the ABO₃ material; f) growing asingle plane of BO₂ upon the alkaline earth oxide film wherein the BO₂of the single plane is epitaxial and commensurate with the alkalineearth oxide of the alkaline earth oxide film; and g) growing a singleplane of AO upon the BO₂ plane so that the metal oxide AO iscommensurate with the BO₂ of the previously-grown BO₂ plane and whereinthe orientation of the grown ABO₃ material is either in a cube-on-cuberelationship with the substrate surface or is rotated 45° with respectto the surface of the substrate so that (001) ABO₃ is parallel to (001)substrate surface and 100! ABO₃ is parallel to 110! substrate surface.3. A monolithic crystalline structure comprising:a semiconductorsubstrate having a material surface provided by a face-centered-cubiclattice structure like that of silicon or silicon-germanium; and anepitaxial film overlying the material surface of the substrate whereinthe epitaxial film includes a perovskite having a lattice parameterwhich closely approximates the quotient of the lattice parameter of thematerial surface divided by the square root of 2.0 and wherein theperovskite has a crystalline form comprised of two constituent metaloxide planes wherein the metal oxide of one of the two constituent metaloxide planes is comprised of TiO₂ so that the Ti metal of the TiO₂ planeprovides a small cation in the perovskite crystalline structure andwherein the metal oxide of the other of the two constituent metal oxideplanes includes another metal which provides a large cation in theperovskite crystalline structure; the perovskite of the epitaxial filmbeing arranged upon the material surface so that a first single planeconsisting of the perovskite constituent TiO₂ is epitaxial andcommensurate with the material surface of the substrate, and a secondsingle plane consisting of the other of the two constituent metal oxideplanes of the perovskite crystalline structure is commensurate with thefirst single plane of TiO₂ and wherein the orientation of the perovskiteof the epitaxial film is rotated 45° with respect to the orientation ofthe material surface of the substrate.
 4. The structure as defined inclaim 3 wherein the epitaxial film includes a layup of perovskitecomprised of a plurality of single planes consisting of TiO₂ and aplurality of single planes consisting of the other of the twoconstituent metal oxides planes of the perovskite crystalline structurewherein the single planes of TiO₂ and the single planes of the other ofthe two constituent metal oxides planes of the perovskite crystallinestructure alternate with one another as a path is traced through thelayup from the material surface of the substrate and each of theaforesaid single planes is commensurate with the corresponding surfaceupon which it overlies.
 5. The structure as defined in claim 4 whereinthe layup of perovskite is at least about twelve cell units in thicknessas measured therethrough from the material surface of the substrate. 6.The structure as defined in claim 3 wherein the perovskite of the filmis a perovskite of the BaTiO₃ class.
 7. The structure as defined inclaim 6 wherein the perovskite of the film is a first perovskite and thestructure further comprises a second perovskite directly contacting andcommensurate with the first perovskite.
 8. The structure as defined inclaim 7 wherein the perovskite of the first perovskite is CaTiO₃ and theperovskite of the second perovskite is BaTiO₃.
 9. The structure asdefined in claim 3 further including a thin film of an intermediatematerial having a sodium chloride-type lattice structure interposedbetween the material surface of the substrate and the film ofperovskite, and said intermediate material directly contacts and iscommensurate with the material surface of the substrate, and the firstsingle plane of the perovskite constituent TiO₂ directly contacts and iscommensurate with the alkaline earth oxide and the second single planeconsisting of the other of the two constituent metal oxide planes of theperovskite crystalline structure is commensurate with the first singleplane of TiO₂ and wherein the orientation of the second single plane isrotated 45° with respect to the orientation of said intermediatematerial.
 10. A process for growing a perovskite film onto the surfaceof a semiconductor-based material wherein the material surface thereofis provided by a face-centered-cubic (fcc) lattice structure like thatof silicon or silicon-germanium, the process comprising the steps of:a)providing an atomically-clean substrate of semiconductor-based materialhaving a material surface which is provided by an fcc lattice structurelike that of silicon or silicon-germanium; b) positioning the substratewithin an oxygen-free environment; c) selecting an alkaline earth oxidehaving a lattice parameter which closely matches the lattice parameterof the material surface of the semiconductor-based substrate; d) growinga film of the alkaline earth oxide upon the material surface wherein thealkaline earth oxide of the film being grown in this step d) isepitaxial and fully commensurate with the material surface of thesemiconductor-based substrate; e) selecting a perovskite having alattice parameter which closely approximates the quotient of the latticeparameter of the material surface divided by the square root of 2.0 andwherein the perovskite has a crystalline form comprised of two metaloxide planes wherein the metal oxide of one of the two metal oxideplanes is comprised of TiO₂ so that the Ti metal of the TiO₂ planeprovides a small cation in the perovskite crystalline structure andwherein the metal oxide of the other of the two metal oxide planesincludes another metal which provides a large cation in the perovskitecrystalline structure; f) growing a single plane of TiO₂ upon thealkaline earth oxide film wherein the TiO₂ of the single plane isepitaxial and commensurate with the alkaline earth oxide of the alkalineearth oxide film; and g) growing a single plane comprised of the otherof the two metal oxide planes of the perovskite crystalline structureupon the TiO₂ plane so that the metal oxide of the other of the twometal oxide planes is epitaxial and commensurate with the TiO₂ of thepreviously-grown TiO₂ plane and wherein the orientation of the grownperovskite is rotated 45° with respect to the material surface of thesubstrate so that (001) perovskite is parallel to (001) material surfaceand 100! perovskite is parallel to 110! material surface.
 11. Theprocess as defined in claim 10 wherein steps involving a growing of asingle commensurate plane of TiO₂ and growing a single commensurateplane of the other of the two metal oxide planes of the perovskitecrystalline structure are repeated in sequence so that a layup ofperovskite comprised of alternating layers of TiO₂ and the metal oxideof the other of the two metal oxide planes is grown upon the materialsurface with commensurate periodicity until a layup of perovskite havinga thickness which is at least as large as the critical thickness of theperovskite is obtained.
 12. The process as defined in claim 12 whereinthe growth of said layup of perovskite is followed by the step ofgrowing an epitaxial and commensurate film comprised of bulk perovskiteupon said layup.
 13. The process as defined in claim 10 wherein theperovskite grown upon the alkaline earth oxide is a perovskite of theBaTiO₃ class.
 14. The process as defined in claim 13 wherein theperovskite grown upon the alkaline earth oxide is Ba₀.75 Sr₀.25 TiO₃.15. A structure formed by the process of claim
 10. 16. A process forgrowing an epitaxial film of a perovskite onto a structure including asubstrate of semiconductor-based material and a film of alkaline earthoxide which directly contacts and is commensurate with the material ofthe substrate, the process comprising the steps of:a) providing astructure including a substrate of semiconductor-based material and afilm of alkaline earth oxide which directly contacts and is commensuratewith the material of the substrate; b) positioning the structure withinan oxygen-free environment; c) selecting a first perovskite having alattice parameter which closely approximates that of the alkaline earthoxide wherein the first perovskite has a crystalline form comprised oftwo metal oxide planes wherein the metal oxide of one of the two metaloxide planes is comprised of TiO₂ so that the Ti metal of the TiO₂ planeprovides a small cation in the perovskite crystalline structure andwherein the metal oxide of the other of the two metal oxide planesincludes another metal which provides a large cation in the perovskitecrystalline structure; d) growing a single plane of TiO₂ upon thesurface of the alkaline earth oxide film wherein the TiO₂ of the singleplane is epitaxial and commensurate with the alkaline earth oxide; ande) growing a single plane comprised of the other of the two metal oxideplanes of the perovskite crystalline structure upon the TiO₂ plane sothat the metal oxide of the other of the two metal oxide planes isepitaxial and commensurate with the TiO₂ of the previously-grown TiO₂plane and wherein the orientation of the single plane grown in this stepis rotated 45° with respect to the alkaline earth oxide upon which theTiO₂ plane of step d) is grown so that the (001) plane of the firstperovskite is parallel to the (001) plane of alkaline earth oxide andthe 100! direction of the first perovskite is parallel to the 110!direction of the alkaline earth oxide.
 17. The process as defined inclaim 16 wherein step e) is followed by the step off) repeating steps d)and e) in sequence so that a layup of the first perovskite comprised ofalternating layers of TiO₂ and the metal oxide of the other of the twometal oxide planes is grown upon the alkaline earth oxide surface untila commensurate layup of the first perovskite of about three cell unitsin thickness is obtained.
 18. The process as defined in claim 17 whereinstep f) is followed by the steps of:g) selecting a second perovskitehaving a lattice parameter which closely matches the lattice parameterof the first perovskite and wherein the second perovskite has acrystalline form comprised of two metal oxide planes wherein the metaloxide of one of the two metal oxide planes of the crystalline form ofthe second perovskite is comprised of TiO₂ so that the Ti metal of theTiO₂ plane provides a small cation in the crystalline structure of thesecond perovskite and wherein the metal oxide of the other of the twometal oxide planes of the crystalline form of the second perovskiteincludes another metal which provides a large cation in the crystallinestructure of the second perovskite; h) growing a single plane of TiO₂upon the other of the two metal oxide planes of the previously-grownfirst perovskite wherein the TiO₂ of the single plane grown in this stepis epitaxial and commensurate with the metal oxide of the surface uponwhich it is grown; and i) growing a single plane comprised of the otherof the two metal oxide planes of the crystalline form of the secondperovskite upon the TiO₂ plane grown in step h) so that the metal oxideof the other of the two metal oxide planes of the crystalline form ofthe second perovskite is epitaxial and fully commensurate with the TiO₂of the TiO₂ plane grown in step h).
 19. The process as defined in claim16 wherein step i) is followed by the step ofj) repeating steps h) andi) in sequence so that a layup of the second perovskite comprised ofalternating layers of TiO₂ and the metal oxide of the other of the twometal oxide planes is grown upon the previously-grown layup of the firstperovskite until a layup of the second perovskite of at least abouttwelve cell units in thickness is obtained.
 20. The process as definedin claim 16 wherein the composition of the alkaline earth oxide uponwhich the first perovskite is grown is Ba₀.725 Sr₀.275 O, thecomposition of the first perovskite is Ca₀.64 Sr₀.36 TiO₃, and thecomposition of the second perovskite is of the BaTiO₃ class.
 21. Theprocess as defined in claim 20 wherein the composition of the secondperovskite is Ba₀.75 Sr₀.25 TiO₃.
 22. A structure formed by the processof claim
 16. 23. In a solid state electrical component including asubstrate of a semiconductor-based material providing a material surfaceprovided by a face-centered-cubic (fcc) lattice structure and a layup ofone perovskite oxide overlying the material surface wherein the oneperovskite oxide has a crystalline form comprised of two metal oxideplanes wherein the metal oxide of one of the two metal oxide planes ofthe crystalline form is comprised of TiO₂ so that the Ti metal of theTiO₂ plane provides a small cation in the crystalline structure of theone perovskite oxide and wherein the metal oxide of the other of the twometal oxide planes of the crystalline form includes a large cation inthe crystalline structure of the one perovskite oxide, the improvementcharacterized in that:the perovskite oxide in the layup of perovskiteoxide is comprised of a plurality of first planes each comprisedentirely of TiO₂ and a plurality of second planes each comprisedentirely of metal oxide wherein the metal of the metal oxide planeincludes the large cation in the crystalline structure of the perovskitestructure, and the first and second planes of the perovskite oxide ofthe layup alternate with one another as a path is traced therethroughfrom the material surface; and an intermediate, epitaxial andcommensurate interfacial film of another perovskite is disposed betweenthe material surface and the layup of the perovskite oxide wherein theperovskite of the interfacial film has a lattice parameter which closelyapproximates the quotient of the lattice parameter of the materialsurface divided by the square root of 2.0.
 24. The improvement asdefined in claim 23 wherein the perovskite of the interfacial film has acrystalline form comprised of two metal oxide planes wherein the metaloxide of one of the two metal oxide planes is comprised of TiO₂ so thatthe Ti metal of the TiO₂ plane provides a small cation in the perovskitecrystalline structure and wherein the metal oxide of the other of thetwo metal oxide planes includes another metal which provides a largecation in the perovskite crystalline structure, andthe perovskite of theinterfacial film is comprised of a plurality of first single planes eachcomprised entirely of TiO₂ and a plurality of second single planes eachcomprised entirely of metal oxide wherein the metal of the metal oxideplane includes the large cation in the crystalline structure of theperovskite structure, and the first single planes and the second singleplanes of the perovskite of the interfacial film alternate with oneanother as a path is traced therethrough from the material surface; andwherein the orientation of the perovskite of the interfacial film isrotated 45° with respect to the material surface so that the (100) planeof the perovskite of the interfacial film is parallel to the (001) planeof the material surface and the 100! direction of the perovskite of theinterfacial film is parallel to the 110! direction of the materialsurface.
 25. The improvement as defined in claim 24 further comprising afilm of alkaline earth oxide interposed between the material surface andthe interfacial film and wherein said film of alkaline earth oxide isepitaxial and commensurate with the material surface.
 26. Theimprovement as defined in claim 25 wherein the semiconductor-basedmaterial is comprised of silicon or a silicon-germanium alloy.
 27. Theimprovement as defined in claim 22 wherein the perovskite oxide of thelayup is adapted to exhibit ferroelectric, piezoelectric, pyroelectric,electro-optic or large dielectric properties during use of thecomponent.
 28. The improvement as defined in claim 27 wherein theperovskite oxide of the layup is adapted to exhibit a high dielectricconstant during use of the component, and the dielectric constant is atleast an order of magnitude greater than that of silica.
 29. Theimprovement as defined in claim 22 wherein the component is aferroelectric field-effect transistor including a base substrate ofsilicon, a source electrode and a drain electrode, a gate electrode anda gate dielectric, and the gate dielectric includes the perovskite oxideof the layup, and the perovskite oxide is interposed between the siliconand the remainder of the gate dielectric, and the interfacial film isdisposed between the perovskite oxide layup and the silicon.
 30. Aferroelectric field-effect transistor including a base substrate ofsilicon, a source electrode, a drain electrode, a gate electrode, and agate dielectric, the improvement characterized in that:the gatedielectric includes an epitaxial and commensurate template film of oneperovskite oxide overlying and commensurate with the silicon and anepitaxial and commensurate thin film of another perovskite oxidedirectly contacting and commensurate with said one perovskite oxidefilm.
 31. The improvement as defined in claim 30 further comprising athin film of an alkaline earth oxide interposed between the surface ofthe silicon and said one perovskite oxide film.
 32. The improvement asdefined in claim 30 wherein said another perovskite oxide is of theBaTiO₃ class of oxides.
 33. A monolithic crystalline structurecomprising:a semiconductor substrate having a surface; and a multilayercommensurate film overlying the surface of the substrate wherein thefilm consists of a first commensurate stratum of single plane layers ofan alkaline earth oxide (AO) and having a sodium chloride-type latticestructure and a second commensurate stratum of single plane layers of anoxide material (A'BO₃) so that the multilayer film arranged upon thesubstrate surface can be designated (AO)_(n) (A'BO₃)_(m) wherein n and mare integer repeats of single plane commensurate oxide layers; andwherein n has been selected to prevent a hydration reaction of the AOstratum with water vapor.